In the field of ink jet recording, water-based inks have been mostly used from the viewpoint of odor and safety. Currently employed water-based inks are solution type inks prepared by dissolving various water-soluble dyes in water or a mixed solvent of water and a water-soluble organic solvent and, if desired, by adding various additives to the solution. An ink jet recording system using these inks has many excellent features including: (1) the recording process is easy in nature of direct recording; (2) the recording process is silent; (3) color recording is easily achieved; (4) high-speed recording is feasible; (5) special paper is not required such that the operating cost is low; and (6) ink is jetted in the form of fine droplets such that an image can be formed with a high resolving power. Because of these advantages, ink jet recording technology is being considered for future recording systems.
However, conventional ink jet recording techniques are disadvantageous in that (1) the ink easily runs on paper (blurring) to deteriorate image quality; (2) the rate of ink drying is low which results in smearing or tailing of the ink on non-image areas (ink stain); (3) the fixing property of the ink is poor; (4) the nozzles and ink passageways tend to become obstructed; (5) the image density is low; and (6) the water resistance is poor.
In order to aleviate these disadvantages, it has been proposed to add a specific surface active agent to an ink composition to thereby reduce the surface tension of the ink and to increase absorption of the ink into the paper as disclosed in JP-A-55-29546 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Furthermore, U.S. Pat. No. 4,352,691 proposes the use of a strongly basic substance to obtain an ink composition having a high pH value which, when transferred onto paper, chemically dissolves a sizing agent added as a waterproofing agent or a pulp to thereby control spread and absorption of ink dots in paper. JP-A-58-13675 teaches the addition of polyvinylpyrrolidone having a molecular weight of 40,000 or more to an ink composition to control spread of dots and absorption in paper. These proposals, however, did not provide a complete solution to the above-described problems.